Devices and methods directed to reducing the moisture content of particulates, e.g. cellulosic material such as wood particles, are utilized in a variety of industries and applications. When wood particles are produced from recently felled trees, the particles contain between 40% and 60% water content. Some industrial processes require wood particles of a far lower moisture content for efficient processing. For example, composition boards, such as particle board, chipboard, and medium density fiberboard (MDF), are extensively used in the construction and furniture industries due to their lower cost and favorable performance in comparison to boards composed of solid wood. However, optimal production of such boards requires wood particles having a significantly reduced moisture content, typically 2% to 10%. As another example, burning of wood particles is a useful means of disposal of waste products from forest industrial processes, such as paper production and sawmills, in a way that extracts and recovers energy. A further example most pertinent to the instant invention is the use of wood particles as an alternative fuel to generate heat and electricity. Particles with a lower moisture content, typically on the order of less than 20%, may be processed more efficiently than "green" particles, as less combustion energy is lost in driving off the entrained moisture.
Prior art devices and methods have addressed the need for drying cellulosic particulate matter, as discussed above, and other divided or particulate materials, such as grains, feed, and food products for human consumption, as well as for dehydration of crystals in the chemical industry. Prior art dryers generally fall into the categories of tumbling convection dryers and convection bed dryers. Tumbling dryers provide for the introduction of wood particles and a drying gas into a drum, wherein the particles are agitated and/or fluidized within the drying gas. When the particles are sufficiently dry for use, they are removed from the drum and separated from the drying gas.
Continuous convection dryers, such as bed dryers, operate by transporting a bed of particles through a convection chamber using conveyors, vibratory decks, paddles, or air jets, while heat is applied to the particles. Some prior art systems provide for agitation and redistribution of particles to increase exposure to the drying means and facilitate uniform drying among the particles. Certain of those embodiments utilize a separate agitating means, but in others the particles are automatically agitated through the action of the transporting means, e.g., devices incorporating vibratory decks, air jets, or multiple vertically aligned conveyors connected by material chutes.
The present technology particular to particle drying has not adequately addressed the needs of some applications requiring dried wood particles or other combustible materials as an alternative energy source to fossil fuels or nuclear fission for power generation. Particularly, present technology is insufficient to satisfy dry fuel requirements of stations requiring large quantities fuel dried with low temperature drying gases. As an example, biomass fueled large-scale sub-stoichiometric, gasification, and pyrolysis systems are currently being developed in many countries for both heat and electricity generation. In the United Kingdom in particular, the market for such developments is dominated by the Non Fossil Fuel Obligation, which has diminished income expectations of fossil fuel powered plants.
Pyrolysis processes demand a continuous supply of dried wood particles in the range of 15% to 20% moisture content. It is anticipated that continuous drying systems will be required to provide as much as 15 to 30 tons of dried wood particles per hour to fuel a single power station. Of critical importance to maximizing the efficiency of electrical generating systems is the availability of fuel feed stock of a consistent quality and moisture level.
The use of low grade heat in wood particle drying systems, generally on the order of 100.degree. C. or less, is effective in maximizing efficiency of such systems and keeping atmospheric emissions to an acceptable level. The most cost effective way of providing this energy is by harnessing waste heat streams from the power plant, either from the exhausts of turbines or engines or from the air cooled condensers on a steam topping cycle. These streams are necessarily of a low grade as most of the useful energy has been used in the process of generating electricity.
Prior art dryers, both of the bed dryer and tumbling dryer variety, have disadvantages which render them inappropriate for use with many power generation systems. Many prior art dryers, particularly the tumbling dryer variety, require the introduction of high-temperature drying gas, typically in the range of 200.degree. to 350.degree. C. Waste heat streams from the power plant do not reach this temperature, and providing additional heat to the gas would substantially increase operating costs. Dryers which operate at these high temperatures are known to cause the release of environmentally unfriendly gases from wood particles and other cellulosic materials. Further, use of such high-temperature gas may cause partial combustion of the wood particles, in turn reducing the energy which may be extracted from the particles in the pyrolysis process;.
Operation of a conventional continuous bed dryer at temperatures in the range obtainable by waste heat would also produce unacceptable results. The increase in drying time necessitated by the low drying gas temperature would call for a prohibitively large drying chamber to maintain the high dry particle output rate required in many power generation systems. Alternatively, an attempt to increase flow rate of the drying gas to increase dry particle output rate would require a prohibitively large fan or blower unit, and would result in uneven drying, with over-drying of some particles in the material bed and insufficient drying of others. Further, an increase in drying gas flow rate could have the detrimental effect of causing particulate entrainment in the drying gas, which may be unacceptable by air quality standards.
Thus, a substantial but unsatisfied need exists for a method and apparatus for drying wood particles using low grade heat.